1. Field of the Invention
The present invention relates to a method of polishing and/or flattening single crystal (monocrystal) or polycrystal diamond, and particularly to a method of polishing and/or flattening diamond so as to provide a diamond substrate having a surface such that it is applicable to various elements, devices and instruments such as cutting instruments, wear resisting instruments, surface acoustic wave (SAW) devices, semiconductor devices, large-area heat sink and diamond lenses.
2. Related Background Art
Diamond not only has an excellent light transmitting property over a wide wavelength range extending from an infrared region to an ultraviolet region except for a part of the infrared region, but also has an excellent pressure resistance, and further diamond is the hardest material among all materials which are present on the earth. Therefore, diamond is the best material for optical use because it is substantially free from scratches.
Further, diamond has the highest Young's modulus among all the materials, so that when a surface acoustic wave is induced, the wave is propagated through diamond at a very high speed. Therefore, diamond has attracted much attention as a material applicable to a high-frequency band-pass filter to be used for a vehicular communication system, etc., and the application thereof is under investigation.
Further, diamond has the largest thermal conductivity among all the materials, and therefore the application thereof to a heat sink for a device such as semiconductor laser and IC has also been investigated.
Moreover, it has been attracted much attention to apply diamond to a material for a device which is stably operable under severe environmental conditions, e.g., at high temperature, under radiation, etc., or is durably operative even at a high output.
A reason for the operativeness of diamond at a high temperature is its band gap as large as 5.5 eV. In other words, intrinsic range, i.e., a temperature range where carriers of semiconductor diamond cannot be controlled is absent at 1400.degree. C. or below.
As a process for artificially synthesizing diamond, an ultra-high pressure synthesizing method has heretofore been employed, but recently diamond crystal has also been synthesized by use of a vapor-phase synthesizing method. Under these circumstances, the application of diamond to optical materials, semiconductor materials, etc., has increasingly been expected.
Recently, in a case where diamond is applied to a surface acoustic wave device, a semiconductor device, a heat sink for various devices or the like, diamond is desired to provide a material having a large area as compared with that in the prior art and having a mirror surface property and/or flatness which is suitable for ultra-fine microfabrication.
However, the surface of artificial diamond produced by the ultra-high pressure synthesizing method or vapor-phase synthesizing method generally has a considerably large roughness of about 1000 .mu.m. Accordingly, in such a case, it is necessary to polish the diamond so as to provide a flat surface.
However, as described above, diamond is the hardest material among all the materials, and it is not easy to polish diamond so as to provide a flat surface.
Heretofore, the surface of diamond has mainly .been polished by using machining (or mechanical process).
Specific examples of such machining include a skiving polishing method wherein a surface of diamond having unevenness is subjected to rough machining by using a physical means such as a grinding machine, then it is polished by using abrasive particles of diamond, and then a diamond component present at the uneven portion on the diamond surface is fused by high-speed friction at a high temperature, thereby to flatten the surface of the diamond.
The conventional polishing method as described above can provide a relatively good mirror surface. However, this method is based on polishing due to direct contact, and therefore the resultant polishing performance largely depends on various conditions or states of a material or substrate to be polished, such as kind of the material, degree of warp, area, thickness of the substrate.
For example, in the case of a diamond film formed on a thin-film silicon substrate, as the area of the diamond film is increased, the warp becomes remarkable, and the degree of the warp may reach several tens of microns in some cases. Accordingly, in such a case, when mechanical pressure is applied to the substrate through the contact polishing, the substrate can be broken. As described above, in the case of the mechanical polishing, there is a certain limit to the dimension or size of a polishing apparatus and a polishing plate, the thickness of a diamond substrate to be polished, etc., and therefore the diamond film which can be subjected to mechanical polishing is practically restricted to one having a small size. Accordingly, it is difficult to mechanically polish a diamond film formed on a large-area (or large-size) substrate.
Japanese Laid-open Patent Application (KOKAI) No. Hei 2-26900 (i.e., No. 26900/1990) discloses a method wherein a metal such as iron is used as a polishing material and a layer of diamond reacted with iron is removed. However, this method still uses a contact-type polishing apparatus, and therefore the above-mentioned problems encountered in the contact polishing are not solved yet.
On the other hand, Japanese Laid-open Patent Application No. Sho 64-68484 (i. e., No. 68484/1989) discloses a method wherein an ion beam, etc., is provided to a surface of diamond in a non-contact mode thereby to flatten the surface of the diamond. The mirror surface property of the diamond can be improved by using such a method. However, the resultant smoothness is decidedly inferior to that obtainable by the contact polishing as described above, and it is impossible to apply the resultant diamond film smoothed by this method to microfabrication, etc.
Japanese Laid-open Patent Application No. 64-62484 (i. e., 62484/1989) discloses a similar method wherein an amorphous carbon film, which is similar to diamond film, is formed on a diamond film by using a CVD (chemical vapor deposition) process, and then the resultant product is subjected to an "etchback" treatment (i.e., the resultant diamond is subjected to etching from the side of the flat amorphous carbon film formed by the above-mentioned manner). However, the diamond surface provided by this method has a considerably large roughness, and sufficient flatness can hardly be obtained. Accordingly, it is difficult to apply the thus provided diamond film to microfabrication. Further, in this method, the formation of the amorphous carbon film having a relatively large thickness consumes a long period of time, and therefore this method is not desirable in view of the production cost.